DQN stuck at suboptimal policy in Atari Pong task
Incase you still haven't been able to resolve the problem, here's a link to the answer to my own question, which has the step-wise changes I made to achieve a +18 average score saturation using just a 10000 replay buffer size and a normal Double DQN (DDQN), trained for about 700-800 episodes. The updated code can also be found here.

No fancy changes like Prioritized Replay Buffer or any secret hyperparameter changes are required. It's usually something simple, like a small problem with the input preprocessing step.